[llvm-dev] [Debuginfo][DWARF][LLD] Remove obsolete debug info in lld.

David Blaikie via llvm-dev
Thu, 04 Jun 2020 18:43:28 -0000

On Thu, Jun 4, 2020 at 8:27 AM Robinson, Paul <paul.robinson at sony.com>
wrote:
>
>
>
> > -----Original Message-----
> > From: David Blaikie <dblaikie at gmail.com>
> > Sent: Wednesday, June 3, 2020 5:31 PM
> > To: Robinson, Paul <paul.robinson at sony.com>
> > Cc: jh7370.2008 at my.bristol.ac.uk; llvm-dev at lists.llvm.org
> > Subject: Re: [llvm-dev] [Debuginfo][DWARF][LLD] Remove obsolete debug
info
> > in lld.
> >
> > On Wed, Jun 3, 2020 at 6:34 AM Robinson, Paul <paul.robinson at
sony.com>
> > wrote:
> > >
> > > DWARF was designed in an era when COMDAT and ICF were not a
thing, or at
> > least not common, certainly not when talking about function code.  The
> > overhead of a unit occurred only once per translation unit, so that
> > expense was reasonably amortized.
> > >
> > >
> > >
> > > Splitting functions into their own object-file sections and
making them
> > excludable is an evolution of compiler/linker technology that DWARF
has
> > not kept up with.  The linker-friendly solutions (COMDAT DWARF) would
put
> > function-related .debug_* contributions into a section-group along
with
> > the function .text itself; this multiplies the total number of
sections to
> > deal with, regardless of the tactics used for the content of each per-
> > function DWARF section.  The fully DWARF-conformant solution would
create
> > one partial_unit per function, with the corresponding overhead of unit
> > headers (especially painful in the .debug_line section).
Alternatively we
> > fragment DWARF into sections without headers and rely on the linker to
> > make everything look right in the linked executable; this produces .o
> > files that are not DWARF conformant (unless we can standardize this in
> > DWARF v6) and would be a big hassle for consumers other than the
linker.
> >
> > "object files don't contain DWARF, but they contain stuff
that the
> > linker will turn into DWARF" wouldn't seem like the worst
thing to me
> > - what sort of pre-linking parsing of DWARF use cases do you have in
> > mind, other than for our own compiler development uses?
>
> No, that wouldn't seem like the worst thing. Obviously llvm-dwarfdump
> would want to be able to report what's actually happening, but indeed
> all the other use-cases that come to mind are not looking at .o files.
>
> > (notwithstanding in-object Split DWARF (where the .dwo sections would
> > have to be remain usable without linking) or the MachO style debug
> > info distribution model which is similar)
>
> I expect Split DWARF would be incompatible with fragments.  I don't
> know details about MachO  but seems likely the same is true there.

Yep, if they're sub-contribution regions, that wouldn't play well with
Split DWARF. (& full contribution isolation have the DWARF header
overhead, etc)

I'd still be concerned about the ELF header overhead even of this
sub-contribution scheme, but could be interesting to see how it plays
out in practice.

All that said, to avoid burying the lede here, I'll splice something
from the end up here:

> Although the point is not to avoid tombstone values, but to do a more
efficient job of editing the final DWARF to omit gc'd functions; it's no
problem at all to use a tombstone value in .debug_addr IMO.

But the tombstone values are Alexey's underlying issue (this ongoing
design discussion for over a year now) & /sort/ of mine too recently
(which, unfortunately, is what's reinvigoraetd this discussion -
would've been nice if I/we/someone had identified this sooner &
could've helped Alexey in a more timely manner): Alexey is dealing
with a platform where 0 is a valid address so the lld/gold strategy of
resolving relocations to dead code to "0+addend" creates ambiguous
DWARF. I'm dealing with a case of zero-length functions ("int f1() {
}" or "void f2() { __builtin_unreachable(); }") causing early
termination of DWARFv4 range lists.

The reason for the DWARF-aware linker proposal was because the "let's
choose a better tombstone" discussion didn't go anywhere & people
sort
of encouraged in this direction of "what if we didn't need a
tombstone/the linker fixed up the debug info instead". So if the DWARF
redundancy elimination doesn't address the issue of zero as a valid
address, it doesn't address Alexey's needs, unfortunately. :/

That said, I super appreciate the time you've put into writing this up
and it is valuable & I'd love to see some (even hand-crafted assembly)
prototypes, maybe do some back-of-the-envelope numbers to see whether
the ELF header overhead would be worth it, etc.

> > But even then, I'm not sure how viable it would be - as Fangrui
> > pointed out on another thread about this: ELF section overhead itself
> > is non-trivial ("sizeof(Elf64_Shdr) = 64.") & it would
probably be
> > rather difficult to reconstruct header-less slice-and-dicable sections
> > in some cases. For type information (a reduced overhead version of
> > -fdebug-types-section) I could see it - but for functions, they need
> > to refer to addresses - preferably in the debug_addr section, and
> > that's accessed by index, so taking chunks out of it would break
other
> > references to it, etc... adding the header would be expensive, and how
> > would the CU construct its DW_AT_ranges value if that has to be sliced
> > and diced? Again, some amount of linker magic might solve some of
> > these problems - but I think there's still a lot of overhead to
making
> > a solution that's workable with a DWARF-agnostic linker (or even
with
> > a DWARF aware one, but in an efficient amount of time/space where
it's
> > not only usable for small programs, or for linking when you're
> > shipping a final production binary, etc)
>
> The idea we have blue-skied internally would work something like this
> (initially explicated in terms of the .debug_info section, then seeing
> how that tactic applies to other sections):
>
> There's a top fragment, containing the CU header and the CU DIE itself.
> Linker magic makes this first in the output file.

Quick curiosity: Is there existing linker magic for this? What does it
look like? I'd love to know so I can play around with hand crafted
prototypes/keep it in mind for such things.

(basically the ability for an object file to say "here's the start and
end of my contribution to this section, and some bits that /can/ go in
the middle, but you can drop them if you like")

> Types also go here; certainly base types, and other file-scope types
> can be included here or put into type units.  (Type units aren't
> fragmented, they are their own thing same as always.)

Separately, it might be worth considering putting types in such a
thing - but, yes, the "How do you reference them when they might be in
your unit or someone else's unit", etc, would have to be figured out.
I guess using an external symbol might be the solution there - again,
with a better understanding of the ^ mentioned linker magic, I'd
probably play around with hand crafting some examples just to see how
this could work.

> There's a matching bottom fragment, which is just the terminating NULL
> for the CU DIE; linker magic makes this last in the output file.

Last of all the contributions from this object file, not last in the
whole output file, right? (please excuse the pedantry, just double
checking)

> Each function has its own fragment, which is in the same link-group
> (COMDAT or whatever) as the function's .text section; that way, if the
> function is discarded, so is the .debug_info fragment.  Offhand I can't
> think of any cases (other than DW_AT_specification, addressed below) of
> references to a subprogram DIE from elsewhere,

The call_site DWARF would want to refer to a subprogram DIE, but that
could be handled by (first pass) having a declaration subprogram in
the initial fragment that the call_site could refer to using the usual
assembler-resolved CU-relative offset. Of course that'd mean a bunch
of (probably the bigger part) of the function's DWARF footprint
wouldn't be deduplicated, but would address this part of the address
tombstone issue (if not using debug_addr) & reduce some of the DWARF -
the addresses are pretty big (if you're not pooling them), etc.

> so it should be fine to
> discard the entire function fragment as needed.  Linker magic puts all
> function fragments between the top and bottom fragments, in some
> indeterminate order.  Each function fragment is the usual complete
> subtree, rooted in DW_TAG_subprogram.

Rooted at the top level (well, below the DW_TAG_compile_unit) DIE, as
you mention later - namespace, or whatever else.

>  References to types are either
> to type units as normal, or to types in the top fragment.  Note that
> these references do not require relocations; type units are by signature
> as always, and for types in the top fragment, the offsets into the top
> fragment are known at compile time.
>
> Inlined functions are described as part of the function they have been
> inlined into, being children of the function DIE.  DW_AT_specification
> refers to the abstract declaration which is in its own fragment (or the
> top fragment, but that keeps the declaration from being elided if all
> references go away).

Yep, this overlaps with the call_site stuff I mentioned earlier - same
ideas. Either top fragment, or its own fragment. Keeping its own
fragment alive, and figuring out how to reference it (depending on
fragment layout/elision) would require some work, but I think it's
do-able. Might even be do-able so it can be deduplicated across CUs
(use a sec_offset form, use a linker-resolved relocation to it) - this
infrastructure would overlap with type deduplication without type
units too.

Though linker resolved relocations add more bytes...

> If functions are inside namespaces, each function fragment will need
> to have namespace DIEs around the function DIE.  This adds overhead
> but it's pretty small.
>
> I hand-wave filling in the CU header's unit length.  I'd expect a
> relocation with a reference to the bottom fragment should be able to
> compute the correct value.

*nod*

> That's the story for .debug_info; what about other sections?
>
> Sections referenced by index from .debug_info can't be fragmented;
> this would be: .debug_abbrev, .debug_addr, .debug_str_offsets.
>
> .debug_str doesn't need to be fragmented, linkers DTRT already.

(linkers deduplicate debug_str - but can they be made to remove
unreferenced strings too? in that cas ewe'd have an interesting
tradeoff of maybe using FORM_strp rather than strx - if we wanted the
linker to be able to drop strings from dropped function definitions,
etc)

> .debug_macro contents are not tied to functions and won't be
fragmented.
>
> .debug_loclists and .debug_rnglists should be fragmentable the same
> way as .debug_info; they exist only as extensions of .debug_info, and
> the range list for the CU itself is merely a concatenated set of
> contributions from each constituent function, so that should Just Work
> (although it won't be optimal, adjacent ranges won't be coalesced).

At least the way we currently emit loclists and rnglists is by using
an index (the header of loclists and rnglists has an index to offset
mapping) - like strx, this would make it hard/impossible for a
DWARF-agnostic linker to see through to find out which indexes were
actually used. We could potentially not use the loclistx/rnglistx
forms/indexes from fragments - instead using sec_offsets that would
make them relocatable/removable/etc. (so long as all the index-based
referenced lists came in the debug_loclist/debug_rnglist header
fragment)

> I believe the same is true for .debug_loc and .debug_ranges, although
> I haven't checked.

Yep, those ones are easier - there's no contribution header, they can
only be referenced via sec_offset, so slicing and dicing them is
cheap.

But the tombstone problem still exists for the CU's debug_ranges -
though /maybe/ it could be carefully constructed from fragments...
that's going to be a /lot/ of sections in the end though.

> .debug_aranges is functionally equivalent to the CU rangelist.

Yup. (as we've touched on before, we don't use aranges at Google -
instead relying on CU's ranges which are just a little more expensive
to retrieve - but no need to duplicate the data in both places - if
consumers really find the aranges worthwhile to avoid parsing a few
attributes on the CU DIE, perhaps a future spec could let
debug_aranges reference a range list? so that aranges and the CU could
share the same data?)

> .debug_line can work the same way as .debug_info but is worth a word.
> The top fragment has the header, including the directory/file lists
> because those are referenced by index.  DW_LNE_define_file can't be
> used.  Each function has a fragment containing the sequence for that
> function, starting with set_address and ending with end_sequence.
> The bottom fragment is empty, existing only to allow the length to
> be computed.

Yep - can't remove dead file and directory names, unfortunately - and
the line table's pretty compact, so not sure it'd be a great savings
(especially compared to the ELF section overhead - at the object file
size at least (though probably a small win for linked executable
size)). Chances are those strings (now in debug_line_str) would be
used /somewhere/ in the program, so linker string deduplication would
get most of the wins - just dead offset entries in the line table
header.

> .debug_line_str is a string section and requires nothing special.
>
> .debug_names ... haven't looked at it but I suspect either it
doesn't
> survive or it has to be generated post-link (or by the linker).

Generally you're going to want a DWARF-aware linker for debug_names,
same as gdb-index, etc.

> .debug_frame I *think* can be fragmented, but I haven't take the
> time to look at it to make sure.
>
> Those are all the sections I see in DWARF v5 Appendix B.
>
> So that's the blue-sky vision of linker-magic COMDAT DWARF, which
> took me about an hour to write down just now.  There is certainly
> a non-trivial overhead in terms of ELF sections; in the general
> case we would have 5 per-function fragments (for .debug_info,
> .debug_line, .debug_rnglists, .debug_loclists, .debug_aranges).
>
> Not small, but then other features in the works are using huge
> quantities of ELF sections too (section-per-basic-block).

That work's being scoped to be fairly selective about which basic
blocks it puts in unique sections - just those that are especially
performance sensitive, so the cost isn't as high as you might
otherwise imagine. Adding 5 new sections per function would be
probably a significantly larger growth than anything else I'm aware
of, but I haven't run the numbers by any means.

Thanks again for the write up!

- Dave

> > & as always, not sure how any of this would work for Split DWARF -
> > just a debug_adr section that has some addresses that point to
> > discardable functions... if we want those addresses themselves to be
> > discardable (so we don't have to use a tombstone value inserted by
the
> > linker) then they'd need to be in separate debug_addr
contributions
> > with headers, etc - the overhead just seems too high to me in all the
> > ways I can look at that.
>
> Yeah I think .dwo sections can't take advantage of fragmenting, and
> .debug_addr is referenced by index so it can't be fragmented.  Although
> the point is not to avoid tombstone values, but to do a more efficient
> job of editing the final DWARF to omit gc'd functions; it's no
problem
> at all to use a tombstone value in .debug_addr IMO.
> --paulr
>
> >
> > >
> > >
> > >
> > > Or we pay the cost of parsing, trimming, and rewriting all the
DWARF in
> > the linker.
> > >
> > > --paulr
> > >
> > >
> > >
> > > From: llvm-dev <llvm-dev-bounces at lists.llvm.org> On
Behalf Of James
> > Henderson via llvm-dev
> > > Sent: Wednesday, June 3, 2020 3:48 AM
> > > To: David Blaikie <dblaikie at gmail.com>
> > > Cc: llvm-dev at lists.llvm.org
> > > Subject: Re: [llvm-dev] [Debuginfo][DWARF][LLD] Remove obsolete
debug
> > info in lld.
> > >
> > >
> > >
> > > It makes me sad that the linker (via a library or otherwise) has
to be
> > "DWARF-aware" to be able to effectively handle
--gc-sections, COMDATs, --
> > icf etc for debug info, without leaving large blocks of data kicking
> > around.
> > >
> > >
> > >
> > > The patching to -1 (or equivalent) is probably a good lightweight
> > solution (though I'd love it if it could be done based on section
type in
> > the future rather than section name, but that's probably outside
the realm
> > of DWARF), as it requires only minimal understanding in the linker,
but
> > anything beyond that seems to be complicated logic that is mostly due
to
> > the structure of DWARF. Patching to -1 does feel a bit like a sticking
> > plaster/band aid to patch over the issue rather than properly solving
it
> > too - there will still be debug data (potentially significant amounts
in
> > COMDAT-heavy objects) that the linker has to write and the debugger
has to
> > somehow know how to skip (even if it knows that -1 is special-case due
to
> > the standard being updated, it needs to get as far as the -1), which
is
> > all wasted effort.
> > >
> > >
> > >
> > > We've already seen from Alexey's prototyping, and from
our own
> > experiences with the Sony proprietary linker (which tried to rewrite
> > .debug_line only) that deconstructing the DWARF so that it can be more
> > optimally reassembled at link time is slow going, and will probably
> > inevitably be however much effort is put into optimising it. For a
start,
> > given the current standards, it's impossible to know how to
deconstruct it
> > without having to parse vast amounts of DWARF, which is typically
going to
> > mean a lot more parsing work than the linker would normally have to
deal
> > with. Additionally, much of this parsing work is wasted effort, since
it
> > seems unlikely in many links that large amounts of the DWARF will be
> > redundant. Having an option to opt-in doesn't help much there,
since it
> > just means the logic exists without most people using it, due to it
not
> > being good enough, or potentially they don't even know it exists.
> > >
> > >
> > >
> > > I don't have particularly concrete suggestions as to how to
solve the
> > structural problems with DWARF at this point. The only thing that
seems
> > obvious to me is a more "blessed" approach to fragmentation
of sections,
> > similar to what I tried with my prototype mentioned earlier in the
thread,
> > although we'd need to figure out the previously stated performance
issues.
> > Other ideas might tie into this, like somehow sharing the various
table
> > headers a bit like CIEs in .eh_frame that could be merged by the
linker -
> > each object could have separate table header sections, which are
> > referenced by the individual .debug_* blocks, which in turn are one
per
> > function/data piece and easily discardable/merged by the linker.
> > >
> > >
> > >
> > > Just some thoughts.
> > >
> > >
> > >
> > > James
> > >
> > >
> > >
> > > On Tue, 2 Jun 2020 at 19:24, David Blaikie via llvm-dev <llvm-
> > dev at lists.llvm.org> wrote:
> > >
> > > On Tue, May 19, 2020 at 7:17 AM Alexey Lapshin
> > > <alapshin at accesssoftek.com> wrote:
> > > >
> > > > Hi David, please find my comments inside:
> > > >
> > > >
> > > > >>>Broad question: Do you have any specific
motivation/users/etc in
> > implementing this (if you can speak about it)?
> > > >
> > > > >>> - it might help motivate the work, understand
what tradeoffs might
> > be suitable for you/your users, etc.
> > > >
> > > > >>There are two general requirements:
> > > > >> 1) Remove (or clean) invalid debug info.
> > > >
> > > > >
> > > > >Perhaps a simpler direct solution for your immediate
needs might be a
> > much narrower,
> > > > >and more efficient linker-DWARF-awareness feature:
> > > > >
> > > > > With DWARFv5, rnglists present an opportunity for a
DWARF linker to
> > rewrite the ranges
> > > > > without parsing the rest of the DWARF. /technically/
this isn't
> > guaranteed - rnglist entries
> > > > > can be referenced either directly, or by index. If all
rnglists are
> > referenced by index, then
> > > > > a linker could parse only the debug_rnglists section
and rewrite
> > ranges to remove any
> > > > > address ranges that refer to optimized-out code.
> > > > >
> > > > > This would only be correct for rnglists that had no
direct
> > references to them (that only were
> > > > > referenced via the indexes) - but we could either
implement it with
> > that assumption, or could
> > > > > add an LLVM extension attribute on the CU that would
say "I promise
> > I only referenced rnglists
> > > > > via rnglistx forms/indexes). If this DWARF-aware
linking would have
> > to read the CU DIE (not
> > > > > all the other DIEs) it /could/ also then rewrite
high/low_pc if the
> > CU wasn't using ranges...
> > > > > but that wouldn't come up in the function-removal
case, because then
> > you'd have ranges anyway,
> > > > > so no need for that.
> > > > >
> > > > > Such a DWARF-aware rnglist linking could also simplify
rnglists, in
> > cases where functions
> > > > > ended up being laid out next to each other, the linker
could
> > coalesce their ranges together.
> > > > >
> > > > > I imagine this could be implemented with very little
overhead to
> > linking, especially compared
> > > > > to the overhead of full DWARF-aware linking.
> > > > >
> > > > >Though none of this fixes Split DWARF, where the linker
doesn't get a
> > chance to see the
> > > > > addresses being used - but if you only want/need the
CU-level ranges
> > to be correct, this
> > > > > might be a viable fix, and quite efficient.
> > > >
> > > > Yes, we think about that alternative. This would resolve our
problem
> > of invalid debug info
> > > > and would work much faster. Thus, if we would not have good
results
> > for D74169 then we
> > > > will implement it. Do you think it could be useful to have
this
> > solution in upstream?
> > >
> > > A pure rnglist rewriting - I think it'd be OK to have in
upstream -
> > > again, cost/benefit/etc would have to be weighed. I'm not
sure it
> > > would save enough space to be particularly valuable beyond the
> > > correctness issue - and it doesn't completely solve the
correctness
> > > issue for zero-address usage or low-address usage (because you
could
> > > still have overlapping subprograms inside a CU - so if you were
> > > symbolizing you could use the correct rnglist to filter, but then
go
> > > look inside the CU only to find two subprograms that had that
address
> > > & not know which one was the correct one an which one was the
> > > discarded one).
> > >
> > > rnglist rewriting might be easy enough to prototype - but depends
what
> > > you want to spend your time on, I know this whole issue has been
a
> > > huge investment of your time already - but maybe this recent
> > > revitalization of the conversation around having an explicit
value in
> > > the linker might be sufficient to address everyone's needs...
*fingers
> > > crossed*)
> > >
> > >
> > > > >> 2) Optimize the DWARF size.
> > > >
> > > >
> > > > > Do your users care much about this? I imagine if they
had
> > significant DWARF size issues,
> > > > > they'd have significant link time issues and the
kind of cost to
> > link time this feature has would
> > > > > be prohibitive - but perhaps they're sharing linked
binaries much
> > more often than they're
> > > > > actually performing linking.
> > > >
> > > > Yes, they do. They also have significant link-time issues.
> > > > So current performance results of D74169 are not very
acceptable.
> > > > We hope to improve it.
> > > >
> > > >
> > > >
> > > > >>The specifics which our users have:
> > > > >>  - embedded platform which uses 0 as start of .text
section.
> > > > >>  - custom toolset which does not support all
features yet(f.e.
> > split dwarf).
> > > > >>  - tolerant of the link-time increase.
> > > > >>  - need a useful way to share debug builds.
> > > >
> > > >
> > > > > Sharing two files (executable and dwp) is significantly
less useful
> > than sharing one file?
> > > >
> > > > Probably not significantly, but yes, it looks less useful
comparing to
> > D74169.
> > > > Having only two files (executable and .dwp) looks
significantly better
> > than having executable and multiple .dwo files.
> > > > Having only one file(executable) with minimal size looks
better than
> > the two files with a bigger size.
> > > >
> > > > clang compiled with -gsplitdwarf takes 0.9G for executable
and 0.9G
> > for .dwp.
> > > > clang compiled with -gc-debuginfo takes only 0.76G for
single
> > executable.
> > > >
> > > >
> > > >
> > > > >>For the first point: we have a problem
"Overlapping address ranges
> > starting from 0"(D59553).
> > > >
> > > > >>We use custom solution, but the general solution
like D74169 would
> > be better here.
> > > >
> > > >
> > > > > If CU ranges are the only ones that need fixing, then I
think the
> > above solution might be as
> > > > > good/better - if more than CU ranges need fixing, then
I think we
> > might want to start talking about
> > > > > how to fix DWARF itself (split and non-split) to signal
certain
> > addresses point to dead code with a
> > > > > specific blessed value that linkers would need to
implement -
> > because with Split DWARF there's
> > > > > no way to solve the non-CU addresses at the linker.
> > > >
> > > > I think the worthful solution for that signal value would be
LowPC >
> > HighPC.
> > > > That does not require additional bits in DWARF.
> > > > It would be natural to skip such address ranges since they
explicitly
> > marked as invalid.
> > > > It could be implemented in a linker very easily. Probably,
it would
> > make sense to describe that
> > > > usage in DWARF standard.
> > > >
> > > > As to the addresses which are not seen by the linker(since
they are in
> > .dwo files) - yes,
> > > > they need to have another solution. Could you show an
example of such
> > a case, please?
> > > >
> > > >
> > > >
> > > > >>>2. Support of type units.
> > > >
> > > > >>>
> > > >
> > > > >>>>  That could be implemented further.
> > > >
> > > > >>>Enabling type units increases object size to
make it easier to
> > deduplicate at link time by a DWARF-unaware
> > > >
> > > > >>>linker. With a DWARF aware linker it'd be
generally desirable not
> > to have to add that object size overhead to
> > > >
> > > > >>>get the linking improvements.
> > > >
> > > > >>
> > > >
> > > > >>But, DWARFLinker should adequately work with type
units since they
> > are already implemented.
> > > >
> > > >
> > > > > Maybe - it'd be nice & all, but I don't
think it's an outright
> > necessity - if someone knows they're using
> > > > > a DWARF-aware linker, they'd probably not use type
units in their
> > object files. It's possible someone
> > > > > doesn't know for sure & maybe they have
pre-canned debug object
> > files from someone else, etc.
> > > >
> > > > I see.
> > > >
> > > > >>Another thing is that the idea behind type units has
the potential
> > to help Dwarf-aware linker to work faster.
> > > >
> > > > >>Currently, DWARFLinker analyzes context to
understand whether types
> > are the same or not.
> > > >
> > > >
> > > > >When you say "analyzes context" what do you
mean? Usually I'd take
> > that to mean
> > > > > "looks at things outside the type itself - like
what namespace it's
> > in, etc" - which, yes,
> > > > > it should do that, but it doesn't seem very
expensive to do. But I
> > guess you actually
> > > > > mean something about doing structural equivalence in
some way,
> > looking at things inside the type?
> > > >
> > > > I think it could be useful for both cases. Currently,
dsymutil does
> > only first thing
> > > > (look at type name, namespace name, etc..) and does not do
the second
> > thing
> > > > (doing structural equivalence). Analyzing type names is
currently
> > quite expensive
> > > > (the only search in string pool takes ~10 sec from 70 sec of
overall
> > time).
> > > > That is expensive because of many things should be done to
work with
> > strings:
> > > > parse DWARF, search and resolve relocations, compute a hash
for
> > strings,
> > > > put data into a string pool, create a fully qualified
name(like
> > namespace::function::name).
> > > > It looks like it could be optimized and finally require less
time, but
> > it still would be a noticeable
> > > > part of the overall time.
> > > >
> > > > If dsymutil starts to check for the structural equivalence,
then the
> > process would be even more slowly.
> > > > So, If instead of comparing types structure, there would be
checked
> > single hash-id - then this process
> > > > would also be faster.
> > > >
> > > > Thus I think using hash-id to compare types would allow to
make
> > current implementation faster and would
> > > > allow handling incomplete types by DWARFLinker without
massive
> > performance degradation also.
> > > >
> > > > >> But the context is known when types are generated.
So, no need to
> > spent the time analyzing it.
> > > >
> > > > >> If types could be compared without analyzing
context, then Dwarf-
> > aware linker would work faster.
> > > >
> > > > >> That is just an idea(not for immediate
implementation): If types
> > would be stored in some "type table"
> > > >
> > > > >> (instead of COMDAT section group) and could be
accessed through
> > hash-id(like type units
> > > >
> > > > >> - then it would be the solution requiring fewer
bits to store but
> > allowing to compare types
> > > >
> > > > >> by hash-id(not analysing context).
> > > > >> In this case, size increasing would be small. And
processing time
> > could be done faster.
> > > > >>
> > > > >> this is just an idea and could be discussed
separately from the
> > problem of integrating of D74169.
> > > >
> > > > >> >> 6. -flto=thin
> > > >
> > > > >> >>    That problem was described in this
review
> >
https://urldefense.com/v3/__https://reviews.llvm.org/D54747*1503720__;Iw!!
> >
JmoZiZGBv3RvKRSx!q8U1OiuTHDnORPTzJINrJOLwncHMDEAyE45t99RrMdkDdSYLjh78mgJen
> > L-N0pxHMQ$ . It also exists in
> > > >
> > > > >> >> current DWARFLinker/dsymutil
implementation. I think that
> > problem should be discussed more: it could
> > > >
> > > > >> >> probably be fixed by avoiding generation
of such incomplete
> > declaration during thinlto,
> > > >
> > > > >> >> That would be costly to produce
extra/redundant debug info in
> > ThinLTO - actually ThinLTO could be doing
> > > >
> > > > >> >> more to reduce that redundancy early on
(actually removing
> > definitions from some llvm Modules if the type
> > > >
> > > > >> >> definition is known to exist in another
Module, etc)
> > > > >> >I don't know if it's a problem since
that patch was reverted.
> > > >
> > > > >>
> > > >
> > > > >> Yes. That patch was reverted, but this
patch(D74169) has the same
> > problem.
> > > >
> > > > >> if D74169 would be applied and --gc-debuginfo used
then structure
> > type
> > > > >> definition would be removed.
> > > >
> > > > >> DWARFLinker could handle that case - "removing
definitions from
> > some llvm Modules if the type
> > > > >> definition is known to exist in another
Module".
> > > > >> i.e. DWARFLinker could replace the declaration with
the definition.
> > > >
> > > > >> But that problem could be more easily resolved when
debug info is
> > generated(probably without
> > > > >> significant increase of debug info size):
> > > >
> > > > >> Here we have:
> > > >
> > > > >> DW_TAG_compile_unit(0x0000000b) - compile unit
containing concrete
> > instance for function "f".
> > > > >> DW_TAG_compile_unit(0x00000073) - compile unit
containing abstract
> > instance root for function "f".
> > > > >> DW_TAG_compile_unit(0x000000c1) - compile unit
containing function
> > "f" definition.
> > > >
> > > > >> Code for function "f" was deleted.
gc-debuginfo deletes compile
> > unit DW_TAG_compile_unit(0x000000c1)
> > > > >> containing "f" definition (since there is
no corresponding code).
> > But it has structure "Foo" definition
> > > > >> DW_TAG_structure_type(0x0000011e) referenced from
> > DW_TAG_compile_unit(0x00000073)
> > > > >> by declaration DW_TAG_structure_type(0x000000ae).
That declaration
> > is exactly the case when definition
> > > > >> was removed by thinlto and replaced with
declaration.
> > > >
> > > > >> Would it cost too much if type definition would not
be replaced
> > with declaration for "abstract instance root"?
> > > > >> The number of concrete instances is bigger than
number of abstract
> > instance roots.
> > > > >> Probably, it would not be too costly to leave
definition in
> > abstract instance root?
> > > >
> > > >
> > > >
> > > > >> Alternatively, Would it cost too much if type
definition would not
> > be replaced with declaration when
> > > > >> declaration references type from not used function?
(lto could
> > understand that concrete function is not used).
> > > >
> > > >
> > > > >I don't follow this example - could you provide a
small concrete test
> > case I could reproduce?
> > > >
> > > > I would provide a test case if necessary. But it looks like
this issue
> > is finally clear, and you already commented on that.
> > > >
> > > > > Oh, I guess this is happening perhaps because ThinLTO
can't know for
> > sure that a standalone
> > > > > definition of 'f' won't be needed - so it
produces one in case one
> > of the inlining opportunities
> > > > > doesn't end up inlining. Then it turns out all
calls got inlined, so
> > the external definition wasn't needed.
> > > >
> > > > > Oh, you're suggesting that these 3 CUs got emitted
into one object
> > file during LTO, but that DWARFLinker
> > > > > drops a CU without any code in it - even though... So
far as I know,
> > in LTO, LLVM directly references
> > > > > types across units if the CUs are all emitted in the
same object
> > file. (and if they weren't in the same
> > > > > object file - then the abstract_origin couldn't be
pointing cross-
> > CU).
> > > >
> > > > > I guess some basic things to say:
> > > >
> > > > > With ThinLTO, the concrete/standalone function
definition is emitted
> > in case some call sites don't end up
> > > > > being inlined. So we know it'll be emitted (but
might not be needed
> > by the actual linker)
> > > > > ANy number of inline calls might exist - but we
shouldn't put the
> > type information into those, because
> > > > > they aren't guaranteed to emit it (if the inline
function gets
> > optimized away, there would be nothing to
> > > > > enforce the type being emitted) - and even if we forced
the type
> > information to be emitted into one
> > > > > object file that has an inline copy of the function -
there's no
> > guarantee that object file will get linked in either.
> > > >
> > > > > So, no, I don't think there's much we can do to
keep the size of
> > object files down, while guaranteeing
> > > > > the type information will be emitted with the usual
linker
> > semantics.
> > > >
> > > > Then dsymutil/DWARFLinker could be changed to handle
that(though it
> > would probably be not very efficient).
> > > > If thinlto would understand that function is not used
finally(and then
> > must not contain referenced type definition),
> > > > then this situation could be handled more effectively.
> > > >
> > > > Thank you, Alexey.
> > > >
> > > >>>
> > > >>>
> > > >>>
> > > >>>
> > > >>> _______________________________________________
> > > >>> LLVM Developers mailing list
> > > >>> llvm-dev at lists.llvm.org
> > > >>>
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> > h78mgJenL-Oh8zYPg$
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